If H = B.t.u.' in 1 Ib. of steam (wet, dry or superheated) leaving the boiler, h = B.t.u. in 1 Ib. of water entering the boiler, then the heat imparted to each pound of steam is H - h and the factor of evaporation is F = (H h) -f- 970.4. The rate of evaporation at normal output is 3,347.9 *-(# A). If W = pounds of water evaporated per hour, the horsepower is W(H - A) + 33,479 = 970.4FT7 + 33,479. If C = pounds of coal burned per hour, W -r C = actual evaporation per pound of coal and FW -r- C = equivalent evaporation per pound of coal. If B = heat -value of coal (B.t.u. per pound) the boiler efficiency is W(H - A) + CB = Q7Q.4FW ^ C5.
The rate of combustion is the weight of coal burned per square foot of grate per hour. It may be expressed by Re = m\/~d, where d = draft between ashpit and furnace, in inches of water, and m = 17 to 32 for anthracite (increasing with the size), 80 and 60 for bituminous run-of-mine and slack, and 50 for run-of-mine semi-bituminous coal.
If 0 = grate area in square feet, C = GRe. Also Re = ^rf^X -gr- In general,
the design ratio R - HS -f- G is high for high rates of combustion of a coal of a given heat value or for good coals at a given rate of combustion. Values approaching 80 give maximum efficiency but imply a costly boiler. Values as low as 30 may advantageously be employed if the load fluctuates greatly. As a rough approximation, W 4- C = 0.00075.
HEAT BALANCE IN BOILER OPERATION (LTJCKE)
Distribution of heat of coal, per cent (D (2) (3)
1. Vaporization of moisture in coal ................. 0.24 0.26 5.00
2. Radiation etc .............................. 4.12 1.98 3.50
3 Unburned gases . . . 0.42 2 17 1 00
4. Fuel in ash and from stack ....... .......... 5.50 18.11 16 00
5 Total furnace loss 10 28 22 52 25 50
6. Sensible heat of stack gases ..................... 10.73 15.19 14 00
7. Radiation, etc .................................. 4.12 1.99 3.50
8. Total surface loss ......... 14.85 17 18 17 50
9 Total loss 25 13 39 70 43 00
10. Heat to steam .... 74 87 60 30 57 00
100.00 100.00 100.00
In the above table, item (10) gives the 'boiler efficiency. The furnace efficiency is 100 10.28 « 89.72 per cent for the first case. The corresponding surface efficiency is (89.72 - 14.85) -f- 0.8972 = 0.834. Then Es X EF = EB or 0.834 X 0.8972 =
Variation in Efficiency with Load.In a given boiler with a given fuel, a straight-line law expresses within limits the relation between the heat imparted to steam and the heat actually developed in the furnace (heat of combustion of the coal, less the furnace losses). It follows that Es decreases as output increases. If EF, as is usual, increases somewhat for increases of load below normal, and